This chapter from CCNP Routing and Switching TSHOOT 300-135 Official Cert Guide discusses common reasons for high CPU and memory utilization on routers and switches, in addition to how we can recognize them. This chapter also covers interface statistics because they sometimes provide the initial indication of some type of issue.

This chapter is from the book

Troubleshooting Switch Performance Issues: This section identifies common reasons why a switch might not be performing as expected.

Troubleshooting Router Performance Issues: This section identifies common reasons why a router might not be performing as expected.

Switches and routers consist of many different components. For example, they contain a processor, memory (volatile such as RAM and nonvolatile such as NVRAM and flash), and various interfaces. They are also responsible for performing many different tasks, such as routing, switching, and building all the necessary tables and structures needed to perform various tasks.

The building of the tables and structures is done by the CPU. The storage of these tables and structures is in some form of memory. The routers and switches forward traffic from one interface to another interface based on these tables and structures. Therefore, if a router’s or switch’s CPU is constantly experiencing high utilization, the memory is overloaded, or the interface buffers are full, these devices will experience performance issues.

This chapter discusses common reasons for high CPU and memory utilization on routers and switches, in addition to how we can recognize them. This chapter also covers interface statistics because they sometimes provide the initial indication of some type of issue.

“Do I Know This Already?” Quiz

The “Do I Know This Already?” quiz allows you to assess whether you should read this entire chapter thoroughly or jump to the “Exam Preparation Tasks” section. If you are in doubt about your answers to these questions or your own assessment of your knowledge of the topics, read the entire chapter. Table 3-1 lists the major headings in this chapter and their corresponding “Do I Know This Already?” quiz questions. You can find the answers in Appendix A, “Answers to the ‘Do I Know This Already?’ Quizzes.”

Table 3-1 “Do I Know This Already?” Section-to-Question Mapping

Foundation Topics Sectio

Questions

Troubleshooting Switch Performance Issues

1-4

Troubleshooting Router Performance Issues

5-8

CAUTION

The goal of self-assessment is to gauge your mastery of the topics in this chapter. If you do not know the answer to a question or are only partially sure of the answer, you should mark that question as wrong for purposes of the self-assessment. Giving yourself credit for an answer that you correctly guess skews your self-assessment results and might provide you with a false sense of security.

What are the components of a switch’s control plane? (Choose two.)

Backplane

Memory

CPU

Forwarding logic

What are good indications that you have a duplex mismatch? (Choose two.)

The half-duplex side of the connection has a high number of FCS errors.

The full-duplex side of the connection has a high number of FCS errors.

The half-duplex side of the connection has a high number of late collisions.

The full-duplex side of the connection has a high number of late collisions.

Which of the following are situations when a switch’s TCAM would punt a packet to the switch’s CPU? (Choose the three best answers.)

OSPF sends a multicast routing update.

An administrator telnets to a switch.

An ACL is applied to a switchport.

A switch’s TCAM has reached capacity.

The output of a show processes cpu command on a switch displays the following in the first line of the output:

CPU utilization for five seconds: 10%/7%; one minute: 12%; five minutes: 6%

Based on the output, what percent of the switch’s CPU is being consumed with interrupts?

10 percent

7 percent

12 percent

6 percent

Which router process is in charge of handling interface state changes?

TCP Timer process

IP Background process

Net Background process

ARP Input process

Which of the following is the least efficient (that is, the most CPU intensive) of a router’s packet-switching modes?

Fast switching

CEF

Optimum switching

Process switching

What command is used to display the contents of a router’s FIB?

show ip cache

show processes cpu

show ip route

show ip cef

Identify common reasons that a router displays a MALLOCFAIL error. (Choose the two best answers.)